This invention relates to protective systems such as used to protect some components of gas turbine engines and, more particularly, to the treatment of the protective-coating surface.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high-temperature durability of the components of the engine must correspondingly increase. Significant advances in high-temperature capabilities have been achieved through the formulation of nickel- and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely employed as a thermal barrier coating (TBC), or topcoat, of TBC systems used on gas turbine engine components. The TBC employed in the highest-temperature regions of gas turbine engines is typically deposited by electron beam physical vapor deposition (EBPVD) techniques that yield a columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation.
To be effective, TBC systems must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between ceramic topcoat materials and the superalloy substrates they protect. To promote adhesion and extend the service life of a TBC system, an oxidation-resistant bond coat is usually employed. Bond coats are typically in the form of overlay coatings such as MCrAlX (where M is iron, cobalt, and/or nickel, and X is yttrium or another rare earth element), or diffusion aluminide coatings. A notable example of a diffusion aluminide bond coat contains platinum aluminide (NiPtAl) intermetallic. When a bond coat is applied, a zone of interdiffusion forms between the substrate and the bond coat. This zone is typically referred to as a diffusion zone. The diffusion zone beneath an overlay bond coat is typically much thinner than the diffusion zone beneath a diffusion bond coat.
During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine service, bond coats of the type described above oxidize to form a tightly adherent alumina (aluminum oxide or Al2O3) layer or scale that protects the underlying structure from catastrophic oxidation and also adheres the TBC to the bond coat. The service life of a TBC system is typically limited by spallation at or near the interfaces of the alumina scale with the bond coat or with the TBC. The spallation is induced by thermal fatigue as the article substrate and the thermal barrier coating system are repeatedly heated and cooled during engine service.
There is a need for an understanding of the specific mechanisms that lead to the thermal fatigue failure of the protective system, and for structures that extend the life of the coating before the incidence of such failure. The present invention fulfills this need, and further provides related advantages.
The present invention provides an approach for fabricating an article protected by a protective system, and articles protected by the protective system. The life of the protective system is extended under conditions of thermal fatigue by delaying the onset of the protective coating/alumina scale convolution failure mode and also by altering the bonding and growth behavior of the alumina scale. The present approach is applicable to environmental-coating protective systems where there is no thermal barrier coating present. However, it realizes its greatest advantages when used in thermal barrier coating systems where the protective coating is a bond coat and a ceramic thermal barrier coating overlies the bond coat.
A method of fabricating an article protected by a protective coating system comprises the steps of providing an article substrate having a substrate surface, and thereafter producing on the substrate surface a protective coating having a polished, pre-oxidized protective-coating surface. The step of producing the protective coating includes the steps of depositing a protective coating on the substrate surface, the protective coating having the protective-coating surface, thereafter polishing the protective-coating surface, and thereafter controllably oxidizing the protective-coating surface. Optionally but preferably, there may be an additional step, after the step of polishing and before the step of controllably oxidizing, of controllably roughening the protective-coating surface. The controlled roughening may be accomplished by any technique, but grit blasting with a fine grit media is preferred. Optionally but preferably, a thermal barrier coating is deposited overlying the polished, pre-oxidized protective-coating surface.
The article substrate preferably is a nickel-base superalloy, and most preferably is a component of a gas turbine engine. The bond coat is preferably a diffusion aluminide bond coat such as a platinum aluminide bond coat. Desirably, the step of polishing the protective-coating surface produces a protective-coating surface that is flattened such that an average grain boundary displacement height of the protective coating is less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably substantially zero, over at least about 40 percent of the grain boundaries of the protective coating but more preferably over the entire grain boundary of the protective coating. Additionally, it is preferred that at least about 40 percent, and more preferably all, of the surface of the protective coating is flattened to have a grain displacement height of less than about 3 micrometers, more preferably less than about 1 micrometer, even more preferably less than about 0.5 micrometer, and most preferably zero.
The step of controllably oxidizing the protective coating preferably includes the step of heating the protective coating in an atmosphere having a partial pressure of oxygen of from about 10xe2x88x925 mbar to about 103 mbar, more preferably from about 10xe2x88x925 mbar to about 10xe2x88x922 mbar, at an oxidizing temperature of from about 1800xc2x0 F. to about 2100xc2x0 F., and for a time of from about xc2xd hour to about 3 hours. Most preferably, the controlled oxidation is performed by heating the protective coating to a pre-oxidation temperature of from about 2000xc2x0 F. to about 2100xc2x0 F. in a heating time of not more than about 45 minutes, preferably from about 1 to about 45 minutes, and more preferably from about 15 to about 35 minutes, and thereafter holding at the pre-oxidation temperature for a time of from about xc2xd hour to about 3 hours, in an atmosphere having a partial pressure of oxygen of about 10xe2x88x924 mbar.
The present approach addresses two major mechanisms of thermal fatigue failure in protective coating systems. The polishing of the protective-coating surface reduces the tendency of the protective coating to form the convolutions that lead to spalling of the alumina that forms on the protective-coating surface. The controlled oxidation of the protective-coating surface improves the bond strength between the protective coating and the alumina scale, and also reduces the growth rate of the alumina scale, so that the oxide reaches its critical thickness after longer times. By forming the alumina scale by a controlled oxidation, the slowly growing alumina scale places less stresses on the bond coat/alumina scale interface. As a result, failure of the protective coating system during thermal fatigue is delayed, improving its life. The optional roughening, as by fine grit blasting, of the protective-coating surface encourages the formation of alpha alumina as distinct from other forms of alumina in the controlled oxidation, and cleans the surface in preparation for the controlled oxidation. Care is taken in the roughening that overly large height displacement defects are not introduced into the surface of the protective coating that would negate the effects of the polishing.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.